Optical connector, optical coupling method and optical element

ABSTRACT

An optical connector used for inputting light to an optical element from an external optical system or outputting light from an optical element to an external optical system includes a photonic crystal having a periodic refractive index structure, and an optical waveguide to be optically coupled to the optical element and a region in which a plurality of defects are formed at intervals equal to or less than four times the refractive index period a of the photonic crystal are formed in the photonic crystal. The region has a size equal to or greater than the wavelength of the light input from the external optical system or the wavelength of the light output to the external optical system, and the external optical system and the optical waveguide are optically coupled to each other via the region. The optical connector has an improved optical coupling efficiency, can achieve optical coupling of a plurality of light components of different wavelengths, and can readily achieve alignment.

BACKGROUND OF THE INVENTION

The present invention relates to an optical connector using a photoniccrystal that can be used for optically coupling an external opticalsystem, such as an optical fiber, and an optical element using aphotonic crystal, an optical coupling method, and an optical elementincorporating the optical connector.

Photonic crystals composed of a periodic array of two or more materialsof difference refractive indices are drawing attention because they canhighly control the behavior of light. Photonic crystals allow light tobe efficiently confined in a spatial domain that is no greater than thewavelength or to be refracted at a steep angle with low loss, so thatthe optical element can be significantly smaller than conventional.Confinement of light can be most efficiently achieved by using athree-dimensional photonic crystal. However, it is difficult for theexisting processing art to precisely fabricate a fine periodic structureon the order of 1 μm, such as a wavelength range for opticalcommunications, and thus, application of a two-dimensional photoniccrystal to various devices is being contemplated.

For example, the periodic refractive index structure of atwo-dimensional photonic crystal is composed of a high-refractive-indexmaterial, such as silicon, and a square lattice array or triangularlattice array of cylindrical holes (holes in the form of a cylinder)made of a low-refractive-index material, such as air, formed therein.Such a periodic structure provides a photonic band gap, and propagationof in-plane light is controlled. On the other hand, the two-dimensionalperiodic structure cannot control vertical propagation of light in adirection perpendicular to the in-plane direction. Thus, thetwo-dimensional periodic structure is made in a slab form, and layers ofa low-refractive-index material, such as air, are provided above andbelow the two-dimensional periodic structure, thereby allowing verticalconfinement of light by the total reflection due to the difference inrefractive index.

If a line defect (formed by eliminating a row of cylindrical holes madeof a low-refractive-index material) is formed in the periodicrefractive-index structure of the two-dimensional photonic crystal slab,there can be provided an optical waveguide that can propagate light withlow loss owing to the periodic structure in the in-plane direction andthe total reflection in the vertical direction. Thus, it is expectedthat an ultra-compact optical integrated circuit can be provided byintegrating optical functional elements, such as an optical waveguideand an optical filter, in a photonic crystal.

In order to put an optical element using a photonic crystal intopractical use, the optical element has to be capable of being opticallycoupled to an external optical system, such as an optical fiber. As alight introducing section of the two-dimensional photonic crystal slab,the line defect optical waveguide described above is popular. As amethod of optically coupling external light to the line defect opticalwaveguide, Japanese Patent Application Laid-Open No. 2001-272555 (issuedon Oct. 5, 2001, referred to as literature 1, hereinafter) discloses anart of externally inputting light perpendicularly to a surface of atwo-dimensional photonic crystal slab, thereby optically coupling thelight to the slab surface.

Specifically, according to the method described in the literature 1, apoint defect that disturbs the periodic refractive index arrangement isformed in the two-dimensional photonic crystal slab, and light is inputto a line defect optical waveguide via the point defect or externallyoutput from the line defect optical waveguide via the point defect. Thepoint defect is formed by changing the diameter of a cylindrical hole(air hole) in the periodic refractive index structure, and light thatcan be input or output via the point defect is limited to light having aparticular wavelength that depends on the shape (diameter) of the pointdefect. Thus, if a plurality of point defects having different shapesare formed in the photonic crystal, light of different wavelengths canbe input to or output from the line defect optical waveguide via therespective point defects.

However, the method of coupling light perpendicularly to the slabsurface via a point defect formed in the two-dimensional photoniccrystal slab described in the literature 1 has a problem that theoptical coupling efficiency is extremely low because the size of thepoint defect (having a diameter of about 0.5 μm, for example) isextremely smaller than the mode size (about 10 μm, for example) of theexternal optical system, such as an optical fiber. In addition, sincethe method is intended for selective coupling of only light of a singlewavelength by controlling the shape of the point defect, there is aproblem that light containing components of different wavelengths fromone optical fiber cannot be coupled to the optical waveguide, forexample. Furthermore, there is a problem that the optical fiber has tobe accurately aligned with the point defect in the plane of thetwo-dimensional photonic crystal slab.

That is, there have not been proposed an optical connector or an opticalcoupling method that improve the optical coupling efficiency, which isreduced because of the difference between the size of the point defectand the light mode size of the external optical system, can couple aplurality of light components of different wavelengths from one opticalfiber to the line defect optical waveguide in the two-dimensionalphotonic crystal slab, and can readily achieve alignment, that is, canmaintain a constant optical coupling efficiency even if the relativeposition of the optical fiber with respect to the two-dimensionalphotonic crystal slab is shifted by several micrometers or more.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical connectorand an optical coupling method that can improve the optical couplingefficiency to an external optical system, such as an optical fiber,which has a large mode diameter, can achieve optical coupling of aplurality of light components of different wavelengths, andsignificantly relax the precision requirement of alignment with anexternal optical system.

According to the present invention, there is provided an opticalconnector used for inputting light to an optical element from anexternal optical system or outputting light from an optical element toan external optical system, comprising: a photonic crystal having aperiodic refractive index structure, in which an optical waveguide to beoptically coupled to the optical element and a region in which aplurality of defects are formed at intervals equal to or less than fourtimes the refractive index period of the photonic crystal are formed inthe photonic crystal, the external optical system and the opticalwaveguide are optically coupled to each other via the region, and theregion has a size equal to or greater than the wavelength of the lightinput from the external optical system or the wavelength of the lightoutput to the external optical system.

According to an optical coupling method according to the presentinvention, in a photonic crystal having a periodic refractive indexstructure, an optical waveguide and a region in which a plurality ofdefects are formed at intervals equal to or less than four times therefractive index period of the photonic crystal are formed, the regionhaving a size equal to or greater than the wavelength of light inputfrom an optical fiber or output to an optical fiber. Then, lightcontaining a plurality of components of different wavelengths is inputto the region from one optical fiber opposed to the region, and theplurality of components of different wavelengths of the incident lightare optically coupled to the optical waveguide from the region.Alternatively, a plurality of components of different wavelengths oflight having propagated through the optical waveguide are opticallycoupled to the region, and the light containing the plurality ofcomponents of different wavelengths is output from the region to oneoptical fiber opposed to the region.

Having the structure described above, the optical connector according tothe present invention has an improved optical coupling efficiency thanconventional when coupling light from an external optical system havinga large mode diameter, such as an optical fiber, to the opticalwaveguide in the photonic crystal. In addition, while only light of asingle wavelength can be optically coupled via a defect conventionally,according to the present invention, light containing a plurality ofcomponents of different wavelengths can be optically coupled via aregion that has an array of a plurality of defects formed therein andhas a size no less than the wavelength of the input or output light.Furthermore, alignment for optical coupling can be achieved more readilythan conventional.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of the present invention(an optical connector composed of a two-dimensional photonic crystalslab in which a line defect optical waveguide and five point defects areformed);

FIG. 2 is a plan view of the optical connector shown in FIG. 1;

FIG. 3 is a graph showing relationships between the incident beam sizeand the optical coupling efficiency to the line defect waveguide;

FIG. 4 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich a line defect optical waveguide and five point defects havingdifferent sizes are formed);

FIG. 5 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich a line defect optical waveguide and five point defects arrangedperiodically along the line defect optical waveguide are formed);

FIG. 6 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich one line defect optical waveguide is adjacent to a region in whichpoint defects are periodically arranged);

FIG. 7 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich one line defect optical waveguide passes through a region in whichpoint defects are periodically arranged);

FIG. 8 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich one line defect optical waveguide is coupled to a region in whichpoint defects are periodically arranged);

FIG. 9 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich two line defect optical waveguides are adjacent to a region inwhich point defects are periodically arranged along the both sidesthereof);

FIG. 10 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich two line defect optical waveguides pass through a region in whichpoint defects are periodically arranged);

FIG. 11 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich six line defect optical waveguides are coupled to a region inwhich point defects are periodically arranged);

FIG. 12 is a plan view of an embodiment of the present invention (anoptical connector composed of a two-dimensional photonic crystal slab inwhich one line defect optical waveguide passes through a region in whichpoint defects are periodically arranged, and two line defects opticalwaveguides are coupled to the same region);

FIG. 13 is a perspective view of the two-dimensional photonic crystalslab shown in FIG. 1 to which light is input from an optical fiber;

FIG. 14 is a plan view showing the position of the beam incident on thetwo-dimensional photonic crystal slab shown in FIG. 6;

FIG. 15 is a graph showing a relationship between the incident beamposition and the optical coupling efficiency to the line defect opticalwaveguide for the structure shown in FIG. 14;

FIG. 16 is a plan view of an embodiment of the present invention (adevice composed of a photonic crystal filter and an optical connectorintegrally formed);

FIG. 17 is a plan view of an embodiment of the present invention (adevice composed of a photonic crystal filter and an optical connectorintegrally formed);

FIG. 18 is a schematic perspective view of an optical module having thedevice shown in FIG. 17 integrated with optical fibers;

FIG. 19 is a perspective view of a two-dimensional photonic crystal slab(an example of the prior art) in which a line defect optical waveguideand one point defect are formed;

FIG. 20 is a plan view of the two-dimensional photonic crystal slabshown in FIG. 19;

FIG. 21 is a plan view of a two-dimensional photonic crystal slab (anexample of the prior art) in which a line defect optical waveguide andtwo point defects of different sizes spaced apart from each other areformed;

FIG. 22 is a plan view of a two-dimensional photonic crystal slab inwhich a line defect optical waveguide and two point defects of the samesize are formed; and

FIG. 23 is a graph showing relationships between the distance betweenpoint defects and the coupling wavelength for different diameters of thepoint defects for the structure shown in FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will bedescribed. In all the embodiments, a two-dimensional photonic crystalslab is used. First, a method of fabricating a two-dimensional photoniccrystal slab will be described step by step.

(1) A silicon-on-insulator (SOI) substrate, which comprises a stack of asilicon substrate, an oxide film overlying the silicon substrate and asilicon film overlying the oxide film, is prepared.

(2) The silicon film on the surface of the SOI substrate is thermallyoxidized to form an oxide film on the surface of the silicon film.

(3) A resist is applied to the oxide film, and the resist is exposed toan electron beam to form a triangular lattice array pattern. The patterncontains a line defect and a plurality of point defects.

(4) The resist is developed, and the oxide film is dry-etched using thedeveloped resist pattern as a mask.

(5) Then, the resist is removed, and silicon film is dry-etched usingthe dry-etched oxide film as a mask. Then, the oxide film is removed bywet etching. In this way, a two-dimensional photonic crystal slab havinga triangular lattice array of cylindrical air holes formed in thesilicon is obtained.

(6) In addition, the base oxide film layer is removed by wet etching. Asa result, there is provided a two-dimensional photonic crystal slab ofsilicon disposed between upper and lower air layers.

Embodiment 1

FIGS. 1 and 2 show a two-dimensional photonic crystal slab that has aline defect optical waveguide and a plurality of point defects (defectholes) in the vicinity thereof. The two-dimensional photonic crystalslab is a silicon slab 11 having a triangular lattice array of air holes(cylindrical holes) 12 formed therein. The period a of the air holes 12,the diameter d₀ of each air hole 12, and the thickness t of the slab 11are as follows.

a=0.42 μm, d₀=0.244 m, and t=0.256 μm

The line defect optical waveguide 13 is formed by eliminating a row ofair holes 12, and five point defects 14 close to each other, whichdisturb the periodic structure of the photonic crystal, are disposed inthe vicinity of the line defect optical waveguide 13. The five pointdefects 14 are arranged in the form of a cross as shown in FIGS. 1 and 2and formed by changing the diameter of air holes 12 in the first, thirdand fifth rows from the line defect optical waveguide 13. The distancebetween adjacent two of the three point defects 14 in the third row istwo-period length (2a=0.84 μm), and the five point defects 14 have adiameter d₁ of 0.47 μm.

Energy levels of the photonic band gap and defects in the photonic bandgap were examined by three-dimensional simulation according to thefinite difference time domain (FDTD) method. As a result, it was proventhat a plurality of peaks occur in the photonic band. Among others, awavelength of λ₁=1.543 μm was selected as an incident light wavelength.A Gaussian beam having a wavelength of λ₁=1.543 μm was input to a region15 of the point defects 14 from a direction perpendicular to thein-plane direction of the two-dimensional photonic crystal slab, and theoptical coupling efficiency of the incident light to the line defectoptical waveguide 13 was assessed. In FIGS. 1 and 2, the arrow 21indicates the incident light, and the arrow 22 indicates output light.The size of the region 15 is larger than the incident light wavelengthλ₁=1.543 μm.

The optical coupling efficiency is the ratio of the total intensity ofthe output light 22 output from the both end faces of the line defectoptical waveguide 13 to the intensity of the incident light 21. Themeasurement was performed by three-dimensional simulation according tothe FDTD method. FIG. 3 shows a result of investigation of therelationship between the beam size (beam diameter) of the incident light21 and the optical coupling efficiency. In the range of beam sizes usedin the measurement, the optical coupling efficiency gently decreased asthe beam size increased. The optical fiber was a single-mode fiber, andthe core diameter thereof was approximately 6 to 10 μm, and the opticalcoupling efficiency was about 0.4% for a beam size of 6.7 μm, which isequivalent to the core diameter of the optical fiber.

Embodiment 2

Unlike the embodiment 1 in which the five point defects 14 have the sameshape, the five point defects 14 have different shapes in thisembodiment 2. Except that the five point defects 14 have differentshapes, a two-dimensional photonic crystal slab was fabricated in thesame manner as in the embodiment 1. FIG. 4 shows a structure of thetwo-dimensional photonic crystal slab.

The point defect 14 located in the first row from the line defectoptical waveguide 13 has a diameter d₁ of 0.49 μm, the three pointdefects 14 located in the third row have a diameter d₂ of 0.47 μm, andthe point defect 14 located in the fifth row has a diameter d₃ of 0.46μm.

As in the embodiment 1, energy levels of the photonic band gap anddefects in the photonic band gap were examined by three-dimensionalsimulation according to the FDTD method. As a result, it was proven thata plurality of peaks occur in the photonic band. Among others, awavelength of λ₁=1.520 μm was selected as an incident light wavelength,and the optical coupling efficiency was assessed as in the embodiment 1.The optical coupling efficiency for a beam size of 6.7 μm was about0.4%. It was proven that, compared with the embodiment 1 in which allthe point defects 14 have the same shape, the coupling wavelength λ₁ canbe changed.

Embodiment 3

In this embodiment 3, the five point defects 14 are arranged in aconfiguration shown in FIG. 5, unlike the embodiment 1 in which thepoint defects are arranged in the form of a cross. Except for thearrangement, a two-dimensional photonic crystal slab was fabricated inthe same manner as in the embodiment 1.

All the point defects 14 have a diameter d₁ of 0.47 μm, and the distancebetween the point defects 14 is L. In this embodiment, the defect levelsformed in the photonic band gap were examined in the same manner as inthe embodiment 1 by changing the distance L from two times (2a=0.84 μm)to seven times (7a=2.94 μm) the refractive index period a of thephotonic crystal.

As a result, it was proven that in the case where the distance L islarger than the four-period length (4a=1.68 μm), the energy levels ofthe defects can be explained by simple superposition of the five pointdefects 14 are formed individually. On the other hand, it was proventhat in the case where the distance L is equal to or less than thefour-period length, the defect energy levels are shifted from the levelsthat would be achieved if the five point defects 14 are formedindividually, and the point defects 14 interact with each other. Inaddition, the optical coupling efficiency was also examined by inputtinga Gaussian beam having a beam size of 6.7 μm. In the case where thedistance L was larger than the four-period length, the optical couplingefficiency was 0.02% (wavelength λ₁=1.587 μm), which is equivalent tothe value in the case where one point defect 14 is formed in an isolatedmanner. And in the case where the distance L was equal to or less thanthe four-period length, the optical coupling efficiency increased as thedistance L decreased, and when the distance L was the two-period length,the optical coupling efficiency was about 0.3% (wavelength λ₁=1.558 μm).

Embodiment 4

As shown in FIG. 6, a line defect optical waveguide 13 and 31 pointdefects 14 were formed in a two-dimensional photonic crystal slabsimilar to that in the embodiment 1. The 31 point defects 14 wereperiodically arranged in a triangular lattice configuration along theline defect optical waveguide 13. The period length of the point defects14 is 0.84 μm (=2a), and all the point defects 14 have a diameter d₁ of0.47 μm. A Gaussian beam was input to a region 15 of the point defects14 from a direction perpendicular to the slab surface. In the case wherethe Gaussian beam had a wavelength λ₁=1.501 μm and a beam size of 2.5μm, the optical coupling efficiency was 0.7%. In FIG. 6, referencenumeral 23 denotes the incident beam. The beam size of 2.5 μmcorresponds to the size of a beam output from a hemispherical-endedfiber, for example.

FIGS. 7 to 9 show examples in which the positional relationship betweenthe region 15 where the point defects 14 are periodically arranged andthe line defect optical waveguide 13 is different from that in FIG. 6.Specifically, unlike FIG. 6 in which the region 15 in which the pointdefects 14 are periodically arranged is adjacent to one line defectoptical waveguide 13, FIG. 7 shows a structure in which one line defectoptical waveguide 13 passes through the region 15. In addition, FIG. 8shows a structure in which one line defect optical waveguide 13 iscoupled to the region 15, that is, an inner end of one line defectoptical waveguide 13 is coupled to the region 15, and FIG. 9 shows astructure in which the region 15 is adjacent to two line defect opticalwaveguide 13 along the opposite sides thereof.

With regard to the structures shown in FIGS. 7 to 9, as in the caseshown in FIG. 6, a Gaussian beam having a wavelength λ₁=1.501 μm and abeam size of 2.5 μm was input to the region 15 substantially at thecenter thereof, and the optical coupling efficiency was examined. Theoptical coupling efficiency was defined as a ratio of the totalintensity of the light output from all the line defect opticalwaveguides 13 (for example, in FIG. 9, the total intensity of the lightoutput from the four ends of the line defect optical waveguides 13) tothe incident light intensity. The optical coupling efficiency was 0.2%for the structure shown in FIG. 7, 0.1% for the structure shown in FIG.8, and 0.4% for the structure shown in FIG. 9.

The positional relationship between the region 15 where the pointdefects 14 are periodically arranged and the line defect opticalwaveguide 13 is not limited to those shown in FIGS. 6 to 9, and anypositional relationships can be adopted as required. FIGS. 10 to 12 showsuch embodiments. FIG. 10 shows a structure in which two line defectoptical waveguides 13 pass through the region 15 in which an array ofpoint defects 14 is formed, and FIG. 11 shows a structure in which sixline defect optical waveguides 13 are coupled to the region 15. FIG. 12shows a structure in which one line defect optical waveguide 13 passesthrough the region 15, and additional two line defect optical waveguides13 are coupled to the region 15. In this way, a plurality of line defectoptical waveguides 13 may pass through, be coupled to and/or be adjacentto the region 15. In FIGS. 10 to 12, the point defects 14 periodicallyarranged in the region 15 are not illustrated specifically.

Embodiment 5

The examination of the defect energy level of the two-dimensionalphotonic crystal slab in the embodiment 1 proved that a plurality ofpeaks occur. Thus, in the embodiment 5, among others, the wavelengths ofλ₁=1.543 μm (the same wavelength as in the embodiment 1) and λ₂=1.581 μmwere selected as the incident light wavelengths, and the opticalcoupling efficiency to the line defect optical waveguide 13 was examinedby inputting a Gaussian beam having wavelengths λ₁ and λ₂ to the region15 in which five point defects 14 were formed. FIG. 13 shows incidentlight 21 having a plurality of wavelengths λ₁ and λ₂ input through oneoptical fiber 31. The two-dimensional photonic crystal slab has the samestructure as in FIG. 1.

The examination result of the relationship between the beam size and theoptical coupling efficiency is shown in FIG. 3. As can be seen from FIG.3, as in the case of the wavelength λ₁=1.543 μm described with regard tothe embodiment 1, in the case of the wavelength λ₂=1.581 μm, the opticalcoupling efficiency gently decreased as the beam size increased withinthe measurement range of beam size. However, it was proven that,regardless of the beam size, the light of both the wavelengths iscoupled to the line defect optical waveguide 13. In the case where thebeam size was 6.7 μm, the optical coupling efficiency of the lighthaving the wavelength of λ₁=1.543 μm was about 0.4%, and the opticalcoupling efficiency of the light having the wavelength of λ₂=1.581 μmwas about 0.15%.

Embodiment 6

Using the two-dimensional photonic crystal slab having the structure inthe embodiment 4 shown in FIG. 6, the optical coupling efficiency ofincident light to the line defect optical waveguide 13 was examined bychanging the position of the incident light along an X direction, wherethe X direction is a direction in which the point defects 14 arearranged along the line defect optical waveguide 13. The incident lightwas a Gaussian beam having a wavelength of λ₁=1.501 μm and a beam sizeof 2.5 μm.

The examination result is shown in FIG. 15. In the region 15 in whichthe point defects 14 are periodically arranged, that is, within a rangeof X=−3.36 μm to X=+3.36 μm, the optical coupling efficiency wassubstantially constant, 0.7%.

Embodiment 7

FIGS. 16 and 17 show embodiments of a device that is composed of aphotonic crystal filter (optical element) constituted by atwo-dimensional photonic crystal slab and an optical connector (opticalcoupling section) constituted by a two-dimensional photonic crystal slabthat are integrally formed. In FIGS. 16 and 17, reference numeral 41denotes a connector section, and reference numeral 42 denotes a filtersection.

In the filter section 42, point defect holes 44 and 45, which serve as aresonant structure, are formed in a line defect optical waveguide 43.The connector section 41 shown in FIG. 16 has the same structure asshown in FIG. 7, in which one line defect optical waveguide 13 passesthrough the region 15 in which point defects 14 are periodicallyarranged, and the line defect optical waveguide 13 is perpendicularlycoupled to the line defect optical waveguide 43 in the filter section42.

On the other hand, in FIG. 17, the connector section 41 has the samestructure as shown in FIG. 9, in which the region 15 in which pointdefects 14 are periodically arranged is sandwiched between two linedefect optical waveguide 13. The two line defect optical waveguides 13are integrated via a Y-shaped line defect optical waveguide 46 and thencoupled to the line defect optical waveguide 43 in the filter section42. The connector section 41 and the filter section 42 of the devicesshown in FIGS. 16 and 17 can be fabricated by a simultaneous process,and therefore, the connector section 41 and the filter section 42 areautomatically aligned with each other with a fine working precision. Inthe following, as an example, the device shown in FIG. 17 will bedescribed.

A Gaussian beam composed of light having a wavelength of λ₁=1.501 μm andlight having a wavelength of λ₂=1.566 μm was input through an opticalfiber to the region 15 perpendicularly to the surface of the photoniccrystal slab. The incident light was coupled to the line defect opticalwaveguides 13 via a plurality of point defects 14 periodically arrangedin the region 15 and guided to the filter section 42. The two light ofdifferent wavelengths λ₁ and λ₂ were separated by the filter section 42,and the light of the wavelength λ₁=1.501 μm and the light of thewavelength λ₂=1.566 μm were output at different positions in the planeof the slab 11, as shown by output light 24 and 25. The connectorsection 41 has multiple point defects 14 periodically arranged, and theoptical coupling efficiency does not vary significantly even if theposition of the optical fiber (position of the incident bean 23) isshifted by several micrometers in the X direction, as described withregard to the embodiment 4. Thus, alignment of the connector 41 can bereadily achieved.

FIG. 18 is a schematic diagram showing a device 40 composed of theconnector section 41 and the filter section 42 shown in FIG. 17 to whichoptical fibers 31 and 32 are attached, which is an optical modulecomprising the device 40 constituted by a two-dimensional photoniccrystal slab, the input optical fiber 31 fixed to the device 40perpendicularly to the slab surface, and the output optical fiber 32fixed to the device 40 in parallel with the slab plane. In FIG. 18,reference numerals 31 a and 32 a denote an optical fiber core.Illustration of the optical fiber for the output light 25 is omitted.The optical fibers 31 and 32 have a diameter D of 125 μm, for example.And for example, the slab-like (rectangular-plate-like) device 40 has asize of A=about 200 μm by B=about 150 μm.

COMPARISON EXAMPLE 1

A two-dimensional photonic crystal slab was fabricated in the samemanner as in the embodiment 1 except that there is formed only one pointdefect. The structure is shown in FIGS. 19 and 20.

The diameter d₁ of the point defect 14 was 0.47 μm, and the point defectwas placed in the third row from the line defect optical waveguide 13.The energy levels of the photonic band gap and the defect in thephotonic band gap were examined by three-dimensional simulationaccording to the FDTD method. As a result, it was proven that a singlepeak occurs at a wavelength λ₁=1.587 μm in the photonic band.

The relationship between the beam size and the optical couplingefficiency to the line defect optical waveguide 13 was examined byinputting a Gaussian beam having a wavelength of λ₁=1.587 μmperpendicularly to the slab surface from above the point defect 14. Theresult is shown in FIG. 3. The optical coupling efficiency rapidlydecreased as the beam size increased, and the optical couplingefficiency was 0.02% when the beam size was 6.7 μm.

COMPARISON EXAMPLE 2

A two-dimensional photonic crystal slab was fabricated in the samemanner as in the embodiment 1 except that there are formed two pointdefects. The structure is shown in FIG. 21.

The diameters d₁ and d₂ of the two point defects 14 and 14′ were 0.47 μmand 0.48 μm, respectively, and the point defects were placed in thethird row from the line defect optical waveguide 13, spaced apart fromeach other by a six-period length (6a=2.52 μm) in parallel with the linedefect optical waveguide 13. The energy levels of the photonic band gapand the defects in the photonic band gap were examined bythree-dimensional simulation according to the FDTD method. As a result,it was proven that a peak due to the point defect 14 occurs at awavelength λ₁=1.587 μm and a peak due to the point defect 14′ occurs ata wavelength λ₂=1.563 μm in the photonic band. The wavelengths λ₁ and λ₂were the same as those in the case where each of the point defects 14and 14′ is formed individually.

The optical coupling efficiency to the line defect optical waveguide 13was examined by inputting a Gaussian beam composed of two kinds of lighthaving a wavelength of λ₁=1.587 μm and a wavelength of λ₂=1.563 μmperpendicularly to the slab surface from above the point defects 14 and14′. When the beam size was 6.7 μm, the optical coupling efficiency was0.01% for both the wavelengths of λ₁=1.587 μm and λ₂=1.563 μm.

From the embodiment 1 and the comparison example 1, the followingconclusions are obtained.

As can be apparently seen from FIG. 3, as the beam size increases, theoptical coupling efficiency gently decreases in the embodiment 1, whilethe optical coupling efficiency rapidly decreases in the comparisonexample 1. Therefore, when the beam size is approximately equal to ormore than 1.5 μm, the optical coupling efficiency is higher in theembodiment 1 than in the comparison example 1. In actual, the corediameter of single-mode optical fibers ranges from about 6 to 10 μm. Insuch a beam size range, a plurality of point defects (defect holes)formed close to each other can provide about ten times higher opticalcoupling efficiency than only one point defect. This can be consideredto be due to the fact that a plurality of point defects formed close toeach other can provide a larger area for interaction with the incidentlight.

In the case where only one point defect is formed as in the comparisonexample 1, the defect energy level appears at the wavelength of 1.587μm. However, in the embodiment 1 in which a plurality of point defectsare formed around the point defect in the comparison example 1, thedefect energy levels do not appear at the wavelength of 1.587 μm, and aplurality of energy levels appear at different wavelengths. It can beconsidered that such a plurality of energy levels appear because thedefect energy levels increases as the number of the point defects, andthe defect energy level that would otherwise appear at 1.587 μmdisappears because the plurality of point defects formed close to eachother interact with each other, so that the defect energy levels aremixed to provide new energy levels.

From the comparison example 2 and the embodiment 5, the followingconclusions are obtained.

In the comparison example 2, light of different wavelengths is coupledto the line defect optical waveguide via the point defects 14 and 14′.In the structure in the comparison example 2, it is necessary that thetwo point defects do not interact with each other. Thus, the two pointdefects are formed at a certain distance from each other. The beam-sizedependency of the optical coupling efficiency of the light of thewavelengths associated with the respective point defects to the linedefect optical waveguide is the same as in the case where each of thepoint defects is formed solely (as in the comparison example 1).Therefore, in the case where light containing two or more components ofdifferent wavelengths is input through one optical fiber, the opticalcoupling efficiency is reduced, compared with the case where a pluralityof point defects are formed close to each other (embodiment 5).

As can be seen from the above description, when a plurality of pointdefects close to each other are used, because of the interaction betweenthe point defects, optical coupling to the line defect optical waveguideoccurs at a plurality of wavelengths, and the optical couplingefficiency for an incident beam size of several micrometers or more ishigher than in the case of only one point defect.

In addition, as can be apparently seen from the embodiment 6, thestructure in which the precision requirement of alignment of the opticalfiber with the photonic crystal having an array of point defects formedtherein is relaxed to no less than the beam size is provided by takingadvantage of the periodic structure of the photonic crystal to form aplurality of point defects close to each other and is proposed for thefirst time by the present invention.

The optical coupling wavelength can be controlled by changing thedistance between or arrangement of point defects, the number of pointdefects, and the combination of the shape of each point defect. Forexample, in the case where two point defects 14 having a diameter d₁ areformed as shown in FIG. 22 (which is the same structure as that in thecomparison example 1 except that two point defects 14 of the samediameter are formed), by reducing the distance L between the two pointdefects 14, the coupling wavelength can be shifted toward shorterwavelengths as shown in FIG. 23. Besides, by changing the diameter d₁the coupling wavelength can be shifted. As an example, FIG. 23 showsrelationships between the distance L and the coupling wavelength in thecases of d₁=0.47 μm, 0.475 μm, and 0.48 μm.

As can be seen from FIG. 23, for example, when a coupling wavelength of1.57 μm is desired, the diameter d₁ of the point defects can be 1.475μm, and the distance L between the point defects can be four-periodlength (4a). While a case of two point defects has been described above,also in the case where three or more point defects are formed, anycoupling wavelength can be achieved by controlling the distance betweenthe point defects or the size of each point defect. In addition, whilethe point defects 14 shown in FIG. 22 are centered on the correspondinglattice points, the centers of the point defects can be shifted from thecorresponding lattice points to control the coupling wavelength. Inaddition, it can also be seen from FIG. 23 that, in the case where thedistance L between the point defects is equal to or greater thanfive-period length (=5a), the coupling wavelength is equal to 1.587 μm(d₁=0.47 μm) in the case where only one point defect is formed(comparison example 1), and interaction between the point defects doesnot occur.

As described above, the positions of the point defects may be displacedfrom the periodic refractive index structure of the photonic crystal,which is a base material. Furthermore, while the two-dimensionalphotonic crystal slab is made of silicon, and the periodic refractiveindex structure is a triangular lattice arrangement in the abovedescription, the material and periodic refractive index structure of thetwo-dimensional photonic crystal slab are not limited to particularones, and any material and periodic refractive index structure can beused as far as there exists a photonic band gap.

In addition, while light is input externally in the embodimentsdescribed above, the same photonic crystal structure and defectarrangement can be used also in the case where light is outputexternally. The size of the region in which a plurality of point defectsare formed is equal to or greater than the wavelength of the incidentlight input from the external optical system. In the case where light isoutput externally, the size of the region in which a plurality of pointdefects are formed is equal to or greater than the wavelength of thelight output to the external optical system.

The optical connector and the optical coupling method according to thepresent invention can be applied to an optical element for long-distancetransmission that places importance on low coupling loss. However, theoptical connector and the optical coupling method according to thepresent inventions can be more suitably applied to an element forshort-distance transmission that is activated by on/off of an opticalsignal which can operate with low light intensity and places importanceon the light intensity being independent of the precision of alignmentwith the optical fiber (that is, low implementation cost).

1. An optical connector used for inputting light to an optical elementfrom an external optical system or outputting light from an opticalelement to an external optical system, comprising: a photonic crystalhaving a periodic refractive index structure, wherein an opticalwaveguide to be optically coupled to said optical element and a regionin which a plurality of defects are formed at intervals equal to or lessthan four times the refractive index period of the photonic crystal areformed in the photonic crystal, said external optical system and saidoptical waveguide are optically coupled to each other via said region,and said region has a size equal to or greater than the wavelength ofthe light input from the external optical system or the wavelength ofthe light output to said external optical system.
 2. The opticalconnector according to claim 1, wherein said plurality of defects areperiodically arranged.
 3. The optical connector according to claim 1,wherein said optical waveguide is coupled to said region at one endthereof.
 4. The optical connector according to claim 1, wherein saidoptical waveguide passes through said region.
 5. The optical connectoraccording to claim 1, wherein said optical waveguide is adjacent to saidregion.
 6. The optical connector according to claim 3, wherein aplurality of optical waveguide are formed.
 7. The optical connectoraccording to claim 4, wherein a plurality of optical waveguide areformed.
 8. The optical connector according to claim 5, wherein aplurality of optical waveguide are formed.
 9. An optical couplingmethod, comprising: a step of forming, in a photonic crystal having aperiodic refractive index structure, an optical waveguide and a regionin which a plurality of defects are formed at intervals equal to or lessthan four times the refractive index period of the photonic crystal, theregion having a size equal to or greater than the wavelength of lightinput from an optical fiber; a step of inputting light containing aplurality of components of different wavelengths to said region from oneoptical fiber opposed to said region; and a step of optically couplingthe plurality of components of different wavelengths of the incidentlight to said optical waveguide from said region.
 10. An opticalcoupling method, comprising: a step of forming, in a photonic crystalhaving a periodic refractive index structure, an optical waveguide and aregion in which a plurality of defects are formed at intervals equal toor less than four times the refractive index period of the photoniccrystal, the region having a size equal to or greater than thewavelength of light output to an optical fiber; a step of opticallycoupling a plurality of components of different wavelengths of lighthaving propagated through said optical waveguide to said region; and astep of outputting the light containing the plurality of components ofdifferent wavelengths from said region to one optical fiber opposed tosaid region.
 11. An optical element that incorporates an opticalconnector according to any of claims 1 to 8.